Tragesser, in U.S. Pat. No. 3,557,876 and Nahm in U.S. patent application Ser. No. 691,905, disclose the addition of pozzolana to drilling fluids. These pozzolanas, when included in drilling fluids, provide a filter cake to prevent fluid loss from the wellbore wherein the filter cake has certain desirable properties. The filter cakes deposited from pozzolana-containing drilling muds can be made to be compatible with cements, and the filter cake itself can be converted into a cementitious material that bonds well with both wellbore cement and the formation. This eliminates problems of contamination of cement by incompatible filter cakes, and eliminates the need to displace mud and filter cake prior to cementing. Pozzolana, according to Tragesser, includes fly ash, flue dust, certain boiler and furnace slags, burnt ground brick, by-products of certain industrial processes, pumicites or volcanic ashes, pumice or pumice stones, obsidian, scoria, tuffs, some of the andesites, diatomites, cherts, shale, clays and pure opal. Nahm discloses particular advantages for the use of blast furnace slag as the pozzolan.
An improvement to the methods of using pozzolan-containing drilling muds is to convert the pozzolan-containing drilling muds directly to cementing slurries. Converting drilling muds directly to cementing slurries eliminates the need to dispose of used muds, eliminates the need to add some materials to the cement that are already present in the muds, such as fluid loss additives, and further improves compatibility of the muds with cementing slurries. Mud-based cements such as these are disclosed in U.S. Pat. Nos. 3,168,139; 3,499,491; and 5,058,679. U.S. Pat. No. 5,058,679 discloses a conversion of drilling mud directly to a cement slurry by adding blast furnace slag; curing agents such as sodium hydroxide, potassium hydroxide, zinc carbonate, or sodium carbonate; and optionally set control additives. The use of blast furnace slag cements instead of Portland cements significantly lowers cement costs, and further improves compatibility with pozzolan-containing drilling muds.
Determination of the amount of blast furnace slag in the drilling mud and in the cement slurry becomes particularly important when blast furnace slag containing drilling mud is converted to blast furnace slag cement or when blast furnace slag containing drilling fluids are used to obtain a settable filter cake. The amount of blast furnace slag remaining in mud will vary depending upon many factors. The amount of blast furnace slag in a final cementing slurry or a recycled drilling mud must be controlled to accomplish an acceptable slurry rheology, set time, and set cement properties.
U.S. patent application Ser. No. 964,981 addresses the need to determine quantitatively the amount of blast furnace slag in a slurry such as a drilling fluid or a cement slurry. This patent provides a method whereby a sample of slurry is annealed to convert the blast furnace slag into crystalline forms that can be quantitatively determined by conventional X-ray diffraction techniques. This method is useful, but it requires a considerable amount of time and requires relatively expensive X-ray diffraction equipment. There remains a need for a quick and simple quantitative test for blast furnace slag in slurries that can be quickly performed at a drilling site.
It is therefore an object of the present invention to provide an analytical method capable of determining the amount of blast furnace slag in slurries such as cement slurries, drilling muds, completion fluids, and workover fluids. It is further object to provide such a method that is simple and can be performed at a drilling site.